The present invention relates generally to communication systems. More particularly, the present invention relates to high-speed communication systems that are particularly well suited for employing multi-carrier modulation schemes.
With the increasing popularity of the Internet, video conferencing and other communication systems that require the transmission of relatively large quantities of data to households and businesses, there have been corresponding demands for higher speed modems for use in bi-directional communications. Given the inherent limitations of single carrier modulation schemes, there has been an increasing interest in the use of multi-carrier modulation schemes.
Some of the more popular systems contemplate the use of digital subscriber lines (e.g. telephone lines), cable lines, and various radio interfaces. In many of the proposed applications, point to point and point to multi-point transmission schemes are contemplated. By way of example, at the time of this writing, the Alliance For Telecommunications Industry Solutions (ATIS), which is a group accredited by the ANSI (American National Standard Institute) Standard Group is working on the next generation subscriber line based transmission system, which is referred to as the VDSL (Very High-Speed Digital Subscriber Line) standard. The VDSL standard is intended to facilitate transmission rates of up to 51.92 Mbit/s.
Simultaneously, the Digital, Audio and Video Council (DAVIC) is working on a short range system, which is referred to as Fiber To The Curb (FTTC). A number of multi-carrier modulation schemes have been proposed for use in the VDSL and FTTC standards (hereinafter VDSL/FTTC). One proposed multi-carrier solution utilizes discrete multi-tone (DMT) signals in a system that is similar in nature to the ADSL standard that was recently adopted by ANSI for a slightly lower speed system. Other proposed modulation schemes include carrierless amplitude and phase modulated (CAP) signals; discrete wavelet multi-tone modulation (DWMT); and orthogonal frequency division multiplexing (OFDM) which is a simplified version of DMT.
A typical subscriber line based telecommunications local loop is illustrated in FIG. 1. As seen therein, a central unit 10 at a central location communicates with a number of remote units R.sub.1 -R.sub.3 22-24 over discrete transmission lines 14, 16 and 18. A variety of transmission media can be used as the transmission line. By way of example, coaxial cables, twisted pair phone lines, and hybrids that incorporate two or more different media all work well. This approach also works well in wireless systems.
The remote units 22-24 may be end user units that may exist in a home, office or the like. Typically, a number of remote units 22-24 are serviced by a particular central office. In currently installed systems, the remote units are often telephones, however, they may be fax lines, computer terminals, televisions, set top boxes or a wide variety of other devices capable of connecting to the "phone lines". The central unit 10 may include a transceiver for each line that functions as a transmitter and a receiver.
In some embodiments, the central unit is a master server located at a central office that originates the communications. In other embodiments, the "central unit" may be a lower level distribution component in the system architecture that receives and retransmits signals. The central unit receives information through a higher bandwidth trunk line and retransmits the information to the remote units. In one embodiment, the trunk line takes the form of a fiber optic cable and the central unit takes the form of an optical network unit (ONU).
The central unit 10 communicates with remote units R.sub.1 -R.sub.3 over discrete lines 14, 16 and 18. Typically, a number of remote units are serviced by a particular central unit or ONU. By way of example, in North America, typical ONUs may service on the order of 4 to 96 remote units. The ONU receives downstream source signals over one or more trunk lines and transmits the information embodied therein to the appropriate remote units as downstream communication signals. Similarly, the ONU receives upstream communication signals from the remote units and transmits the information embodied therein as upstream source signals. The source signals may be passed to a central office, another distribution unit or any other suitable location. A service provider would typically be arranged to provide the data to the central modem for transmission to the remote units and to handle the data received by the central modem from the remote units. The service provider can take any suitable form. By way of example, the service provider can take the form of a network server. The network server can take the form of a dedicated computer or a distributed system.
The distance between the central unit 10 and the furthest remote unit may vary a fair amount. By way of example, it is expected that in the VDSL/FTTC standards, twisted pair loop lengths of up to 1000 feet (300 meters) will be permitted for downstream communications at 51.92 MHz. Similarly, loop lengths of up to 3000 feet (900 meters) may be permitted for downstream communications at 25.96 MHz and loop lengths of up to 5000 feet (1500 meters) may be permitted for downstream communications at 12.97 MHz. As will be appreciated by those skilled in the art, shorter maximum loop lengths generally correspond to higher achievable data rates.
A problem associated with having remote units at different distances from the central unit is the occurrence of non-uniform far-end crosstalk, often referred to as "near-far FEXT" or "unequal-level FEXT". Most models used to design and optimize point to multi-point communication systems assume that the amount of cross-talk between lines (e.g., 14, 16 and 18) is the same between all the lines. That is, the models assume that all the lines are of equal length.
Referring back to FIG. 1, the drop points of the remote units 22-24 are located at distances d.sub.1, d.sub.2 and d.sub.3. In a typical communication system all the remote units transmit at the maximum possible power because all the remote units believe they are equally distant from the central unit 10.
FIGS. 2a-2c, 3a-3b and 4 illustrate the power spectral densities (PSDs) of the transmissions of the remote units at distances d.sub.1, d.sub.2 and d.sub.3. Referring to FIG. 2a, the output of remote unit R.sub.1 is the maximum power output over the entire usable frequency range, 0&lt;f&lt;f.sub.i, at distance d.sub.1, the drop point of the remote unit. In FIG. 2b, the signal attenuates over distance at d.sub.2 according to the transfer function H(f) of line 14. The signal further attenuates at distance d.sub.3, as shown in FIG. 2c. The attenuation of the signal from remote unit R.sub.1 to central unit 10 is a normal function of the transmission medium.
FIGS. 3a and 3b depict the PSD of a signal transmitted from remote unit R.sub.2 to central unit 10 at distance d.sub.2 and d.sub.3, respectively. At point d.sub.2 the output of remote unit R.sub.2 is the maximum over all the usable frequency range, as shown in FIG. 3a. FIG. 3b illustrates the attenuation of the signal at point d.sub.3. Notice that the PSD of the signal on line 14 at point d.sub.2 is significantly less than the PSD of the signal of line 16 at point d2. Again, most models and designs assume that the signal strengths of two signals at any given point is the same, thus causing the same amount of cross-talk. In reality, the signal carried on line 16 will cause more cross-talk interference on line 14 than the signal on line 14 causes on line 16.
The problem is exacerbated with more remote units at varying distances. FIG. 4 illustrates the PSD of a signal transmitted from remote unit R.sub.3 on line 18 to central unit 10 at distance d.sub.3. The PSDs of the signals carried on lines 14, 16 and 18 are significantly different. The difference in the signal strengths leads to more cross-talk interference generated by the shorter lines. In the illustrated embodiment, the signal carried on line 18 would tend to cause cross-talk interference in both lines 14 and 16.
In typical systems no account is made for the different signal strengths of signals carried on different length lines. Most systems assume that all the remote units are equidistant from the central unit, and therefore the amount of cross-talk caused by other lines n a particular are the same as the cross-talk generated by that particular line. As illustrated, in most cases this assumption is incorrect. Thus, when near-far FEXT occurs the throughput of the lines are degraded and the communication system becomes less efficient than expected. Therefore, a need exists for improved methods of minimizing the effects of near-far FEXT.